Method of making a microwave element



Dec. 7, 1965 J. w. NlELsEN METHOD OF MAKING A MICROWAVE ELEMENT FiledMay 17, 1962 3,221,395 METHD @E MAKING A MECRGWAVE ELEMENT .iarnes W.Niel-sen, Beiheley Heights, NJ., assigner to Airtron, ine., MorrisPlains, NJ. Fiied May i7, 1962, Ser. No. 195,625 6 Claims. (Cl. 29-1555)The present invention relates to ferromagnetic materials, and moreparticularly to ferromagnetic materials employed in microwave devices.

In the field of microwave limiters and filters, the use of ferromagneticmaterials such as ferrites has been proposed heretofore. One technicalarticle which relates to this subject matter is entitled Low TemperatureMicrowave Power Limiter, by F. l. Sansalone and E. G. Spencer, page 272of the May i961 issue of the I.R.E. Transactions on Microwave Theory andTechniques. In the limiters disclosed in the prior art, suitableferromagnetic materials are employed as the coupling element between twomicrowave circuits to produce a form of saturation, and this nonlineareffect provides a relatively constant output for varying signal inputlevels.

In accordance with the teachings of Harry Suhl in FerromagneticResonance at High Signal Powers, Journal of Phys. and Chem. of Solids,vol. 1, pp. 209-227, April 1.7957, it is particularly important to havea relatively narrow resonance line in order to get low level limiting.Various techniques for reducing the width of the resonance line haveinclu-ded polishing the surface, and heat treatment. While these twotechniques have been found helpful, prolonged grinding or extendingheating have actually been found to produce adverse effects, in that theline width is increased rather than progressively decreased.

A principal object of the present invention is to decrease the resonanceline width -of ferromagnetic materials. A collateral object of thepresent invention is to produce an improved microwave limiter byreducing the microwave signal threshold level for the onset ofnonlinearity and limiting.

In accordance with the present invention, it has been determined thatthe increase in line width resulting from progressive grinding isprobably a result of growth induced crystal anisotropy. Thus, in thegrowth of crystals, certain slight changes in composition occur. As thelayers are progressively ground oif the outer surface of the crystal,the line width increases and decreases in a cyclic manner as slightchanges in the surface composition take place. Furthermore, inaccordance with the present invention, it has been determined that theincrease in line width following heat treatment of certain ferromagneticmaterials is due to the loss of volatile components which are present inthe ferrites. Thus, heating has the desired effect of relieving strainbut produces the adverse effect of changing the chemical compositionthrough the loss of the volatile component. This adverse effect can beinhibited by providing the sphere with an excess of the volatilecomponent in question during the heating or annealing step. Under thesecircumstances, the line width is reduced by strain relief and nocounterbalancing increase in line width from chemical compositionchanges occurs.

One important limiter material is lithium ferrite. The volatile natureof the lithium has produced an undesired increase in line width uponheat treatment, as noted above. By coating with any suitablelithium-containing solution or slurry, the loss of lithium which wouldotherwise occur upon heating, is prevented. It is contemplated that`other ferromagnetic materials containing volatile components can behandled in a similar manner to prevent changes in chemical composition.

In accordance with a feature of the invention, an elezih Patented Bee.'7, i965 ment of microwave ferromagnetic material having at least onevolatile component is ground, and is subsequently heat-treated in thepresence of an excess of the volatile component to relieve strain andincrease the uniformity of the crystalline structure. A collateralfeature involves the use of the resultant substantially isotropic andstrainfree 'element in a microwave device, particularly for improvedlow-level limiting.

Through the use of the present techniques, microwave resonance linewidths of less than three oersteds have been obtained, as contrastedwith values of six oersteds or more, which was the lowest value that hadbeen reported previously for spinel ferrites.

Other objects, features and advantages of the invention will becomeapparent upon consideration of the following detailed description whentaken in combination with the accompanying drawings.

In the drawings:

FIGS. 1 and 2 are plots of the change in ferromagnetic resonance linewidth with grinding and heat treatment, respectively; and

FIG. 3 is a schematic showing -of a microwave limiter employing aferromagnetic coupling element treated in accordance with the principlesof the present invention.

In accordance with the article by Harry Suhl cited above, the expressionfor the critical held for the onset of nonlinear effects is as follows:

where AH is the line width, AHk the spin wave line width, and MMS is thesaturation magnetization. Since 11m, should be as small as possible forlimiting, AH should be as small as possible.

In the course of one illustrative implementation of the principles ofthe present invention, lithium ferrite crystals were grown from moltenlead oxide solutions which were cooled slowly from l250 C. Chemicalanalyses of the crystals showed a formula of Li0 51Fe25O4. The directcurrent resistivity of the crystals was about ohm-centimeters. At roomtemperature, the saturation magnetization, designated 41rMS, was equalto 355040 gauss.

Rough ground spheres of the crystalline lithium ferrite had aferromagnetic resonance line width which were more than 40 oersteds. Theline width measurements were made at five kilomagacycles at roomtemperature, using a technique in which the sphere orients itself in anapplied magnetic field, as described by R. C. LeCraw and E. G. Spencer,Line Width Narrowing in Gallium Substituted Yttrium Iron Garnet,Bulletin of American Physical Society, p. 85, January 1960. Uponpolishing the :sphere with successively finer grits, the line widthdecreased. After four or five hours of polishing, diamond grit having anaverage particle size of 0.25 micron was employed. Further polishingdecreased the line width to about 7.5 oersteds. With prolongedpolishing, the line width rose and fell in a periodic manner as shown inFIG. l of the drawings.

The plot of FIG. l is typical of plots which were obtained by a grindingof many other lithium ferrite crystals of slightly differentcomposition. In each case the line width varied with progressivepolishing and in most instances a minimum value of about 7.5 oerstedsfor the line width was obtained.

Grinding with the finest aluminum oxide grit, having a chemical formulaof A1203 and a particle size of about 0.3 micron, appeared to increasethe line width, although the peniodiv change was still observed. Withreference to the curve of FIG. 1, the section of the characteristicbetween point 12 and point 14;- was ground using the diamond grit, andthe section of the curve to the right of point 14 was obtained with thealuminum oxide grit.

The switch to aluminum oxide was made to improve the A surface finish,as the aluminum oxide is softer and has more uniform particle size thanthe diamond grit.

The crystals at the rst polishing minimum were pitted in an irregularmanner. Yttrium iron garnet in the same state of polish exhibits a linewidth of several oersteds.

FIG. 2 illustrates the effect of heat treatment -at 800 C. of a groundand polished sphere of lithium ferrite. Following about five hours ofheat treatment in air, a minimum line width of about 5 oersteds wasobtained. This figure was typical of the curves which were obtained formany additional spheres. In nearly every case, either in air or oxygen,the line width decreased to about the same value, regardless of theinitial value. This data holds for all of the spheres which had beenpolished to or beyond the first polishing minimum. The average minimumline width was about 2 oersteds less than the minimum line widthobtained by polishing. The lack of difference in this minimum betweensamples treated in oxygen and those treated in air indicates that thedecrease in line width does notv depend significantly on the oxidationof divalent iron at the surface.

Treatment at 800 C. permits the mobile lithium ions to smooth out theslight periodic irregularities in the composition of the ferrite whichare present in the form in which it is crystallized from the melt, andthe heat treatment simultaneously relieves strain through annealingaction. Further polishing verifies these facts, in that the resonanceline width varies but slightly with additional polishing after the heattreatment. The slight further variations are attributable to Variationsin strain caused by the further polishing action.

The increase in resonance line width with prolonged heating at 800 C. inthe absence of a coating solution is principally attributable to thevapoizat-ion if lithium. This loss is corroborated by the decrease inresonance line width which occurs with heat treatment when the sphereshave been previously coated with a solution or a slurry of alithium-containing substance.

With regard to the solutions which may be employed for treating thelithium, any suitable solution containing available lithium may beemployed. A water solution of lithium carbonate has been employedsuccessfully. Other suitable lithium salts which may be put intosolution or into a slurry for application to the sphere prior to heattreatment include lithium nitrate and lithium chloride. Solvents otherthan water may, of course, be employed, with due care being taken in thecase of inammaible solvents.

Following heat treatment in the presence of an excess of lithium, thelithium ferrite element is anealed and substantially strain-free. Inaddition, it has a concentration of lithium which is substantially thesame at its surface as just within its surface. This substantiallyisotropic composition, in combination'with the strain-free state of theferrite element, are believed to account for the low resonance linewidth which is achieved.

FIG. 3 represents a typical limiter junction in which signals from oneof the transmission lines 22 are coupled to the other transmission line24 by the ferrite sphere 26 of improved material. The structu-re of FIG.3 represents a pair of crossed coaxial waveguides having conductors 22and 24 within the rectangular waveguide channels 28 and 30,respectively. Insulating supporting elements 32 and 34, which may, forexample be made of polyfoam, hold the conductors 22 and 24, as well asthe sphere 26 in their proper relative positions.

The sphere 26 must be magnetically biased with a steady magnetic fieldoriented perpendicular to the two transmission lines 22 and 24. This mayreadily be accomplished by a pair of magnets 36 and 38. The magnetstructure may have a closed external loop to complete the magneticcircuit; it may also be a permanent magnet 4 arrangement` instead of theelectromagnets shown in FIG. 3.

With regard to the temperatures for heat treatment, a temperature ofabout 800 is preferred for lithium ferrite annealiny or strain relief.However, temperatures from 720 to 900 may be used. At lower temperaturesthe annealing proceeds more slowly, and at higher temperatures excessivechemical changes of the ferrite may occur.

Concerning materials, the lithium ferrite advantageously has acomposition near the Li0 5Fe2.5O4 proportions set forth hereinabove.However, limiter action may still be obtained with substantialdepartures from these proportions. Thus, for example, the amount oflithium in the lithium ferrite may range from that given by the formulaLi0,1Fe2,95O4 to much greater proportions of lithium, such as Li1,5Fe22O4, for example. Microwave materials containing other volatilecomponents such as gallium, in addition to or instead of lithium, mayalso be employed. The use of an excess of the volatile component, orcomponents, in the course of heat treatment. will avoid changing thecharacteristics of the material by chemical composition changes.

Another process known as ordering may also be :used to further reducethe resonance line width to a minor extent. Ordering involves thelregularity of the distribution of substituents in a crystal lattice. Inthe case of lithium ferrite having the formula (Fe) [Li,5Fe1 5]O4,Vwhere the pa-rentheses indicate location in the tetrahedral sites ofthe spinel crystal lattice and the brackets indicate location in theoctahedral sites of the spinel, the lithium atoms share the octahedralsites with the iron atoms. It had previously been determined that anarrangement wherein the lithium atoms were regularly spaced betweensuccessive sets of three iron atoms has a slightly lower resonance linewidth than other less ordered arrangements. This ordering occurs mostrapidly at temperatures below and close to 700 C. At temperatures above700 C., the crystal assumes a disordered state. Accordingly, followingthe heat treatment at 800 C. for strain relief, a subsequent heattreatment close to 700 C. is performed for ordering.

For completeness, it is interesting to note that the frequency range formicrowave materials is roughly proportional to the saturationmagnetization. Thus, the useful frequency range for yttrium iron garnet,having a saturation magnetization of about 1750 gauss, is about 2,000 to4,000 megacycles; While the useful frequency range for llithium ferrite,having a saturation magnetization of about 3,500 to 3,900 gauss, isabout 5,000 to 7,000 megacycles. For this last mentioned high frequencyrange the present method has produced the lowest resonance line widthswhich have been obtained up to the present time.

It is to be understood that the above-described arrangements areillustrative of the application of the principles of the invention.Numerous other arrangements may be devised by those skilled in the artwithout departing from the spirit and scope of the invention. Thus, byway of example and not of limitation, certain aspects of the inventionare applicable to other microwave materials which include volatilecomponents, such as gallium doped yttrium iron garnet or lithium ferriteincluding some low melting point gallium material. In addition, thecoupling element may be ellipsoidal, in the form of a disc or have othershapes depending on the geometry and other excitation requirements ofthe unit. It may also be noted that the annealing and orderingtemperatures set forth in the body of the specification are for oneparticular substance having a specific composition; these temperaturesmust be altered for other materials and compositions in accordance withknown criteria to produce the same results. Accordingly, from theforegoing remarks, it is understood that the present invention is to belimited only by the spirit and scope of the appended claims.

What is claimed is:

1. A method for reducing the ferromagnetic resonance line width of anelement of lithium ferrite material comprising:

grinding and polishing the element with fine abrasive grit;

applying material containing available lithium to the entire outersurface of said element; and

annealing the element while it is in contact with the applied material.

2. A method for treating an element of ferrite material having avolatile component comprising:

polishing the element with ne abrasive grit;

coating the ferrite material with a solution containing said volatilecomponent; and

annealing the ferrite material while it is covered with the solution.

3. A method for treating an element of microwave material having avolatile component comprising:

polishing the element with line abrasive grit;

contacting the entire surface of said element with an excess of saidvolatile component;

annealing the element while it is in contact with the excess of saidcomponent; and

ordering the material of said element at a temperature below theannealing temperature.

4. A method for treating an element of lithium ferrite comprising:

polishing the element with ne abrasive grit;

contacting the entire outer surface of said element with an excess oflithium;

annealing the element while it is in contact with the lithium; and

ordering the lithium ferrite element at a temperature below theannealing temperature.

5. A method for making a microwave element for use in the kilomegacyclefrequency range comprising:

grinding and polishing an element of lithium ferrite having a chemicalcomposition between Li0 1Fe2,95O4 and LT15'F62'2O4;

applying a liquid containing available lithium to the element;

annealing said element at a temperature between 720 C. and 900 C.; and

maintaining the lithium ferrite at a temperature near 700 C. to orderthe material.

6. A method for making a microwave element for use in the kilomegacyclefrequency range comprising:

grinding and polishing an element of lithium ferrite having a chemicalcomposition LimFez'gO.; and L1.5F2.2O4;

applying a liquid containing available lithium to the element; and

annealing said element at a temperature between 720 C. and 900 C.

References Cited by the Examiner UNITED STATES PATENTS 2,886,530 5/1959Greger 252-625 2,920,292 l/ 1960 Scovil 333-24 2,922,125 1/1960 Suhl333--24 3,038,860 6/1962 Vinal et al. 252-625 3,093,588 6/1963 Brown252-62.5

FOREIGN PATENTS 167,179 4/1954 Australia. 1,242,007 8/1960 France.

OTHER REFERENCES IRE Transactions on Microwave Theory and Techniques,May 1961 (pages 272-273).

JOHN F. CAMPBELL, Primary Examiner.

HERMAN K. SAALBACH, Examiner.

1. A METHOD FOR REDUCING THE FERROMAGNETIC RESONANCE LINE WIDTH OF ANELEMENT OF LITHIUM FERRIE MATERIAL COMPRISING: GRINDING AND POLISHINGTHE ELEMENT WITH FINE ABRASIVE GRIT; APPLYING MATERIAL CONTAININGAVAILABLE LITHIUM TO THE ENTIRE OUTER SURFACE OF SAID ELEMENT; ANDANNEALING THE ELEMENT WHILE IT IS IN CONTACT WITH THE APPLIED MATERIAL.